PROJECT SUMMARY Background: Sex Chromosome Trisomies (SCT) including Klinefelter (XXY), Trisomy X (XXX), and XYY syndromes occur in 1 out of every 500 births and are associated with a broad phenotypic spectrum including increased risk for developmental delays (DD), language/learning disorders, and autism spectrum disorder (ASD). XXY is also associated with testicular failure, XXX increases risk for ovarian failure, and disorders of insulin resistance and other medical problems resulting in increased morbidity and mortality occur in all 3 SCTs. Historically, less than 10% of SCT diagnoses occur in childhood, however the rate of newborns with SCT has markedly increased with new noninvasive prenatal cell?free DNA (cfDNA) screening. SCT natural history research is limited to studies from the 1970's, and we have little knowledge of early predictors of the wide heterogeneity in later outcomes. Increasing research suggests that androgen therapy during infancy in XXY may improve developmental and health outcomes, supporting the need for newborn screening so intervention can be delivered during this critical period. The very high risk for DD in SCT also suggests that newborn screening may improve timely initiation of interventions. However, it is not clear whether all SCT infants indeed require intensive developmental assessments and therapies, or if primary care screenings are sufficient to identify those in need. The surge in prenatal SCT diagnoses from cfDNA methods provides an opportunity for longitudinal study of a cohort of infants to explore natural history, and to improve care. Aims: This study aims to: (1) describe and compare the natural history of neurodevelopment, health and early gonadal function in infants with the 3 SCT conditions through a national prospective eXtraordinarY Babies Study in partnership with the Newborn Screening Translational Research Network (NBSTRN), (2) identify early predictors of poor neurodevelopmental and cardiometabolic outcomes, and (3) evaluate the sensitivities of common primary care developmental screening measures to detect DD and ASD in this high?risk population to inform recommendations for an early neurodevelopmental care protocol. Approach: Infants with a prenatal diagnosis of XXY (n=100), XYY (n=50), or XXX (n=50) will be followed prospectively every 6?12 months for 2?4 years at 2 eXtraordinarY Kids Clinic sites. Demographics, health history, development, interventions, and social/family history will be collected using NBSTRN common data elements. Assessments will include: (1) measures of cognitive, language, social, motor, and adaptive function, (2) physical exam, gonadal function labs, cardiometabolic measures, and body composition, and (3) quality of life outcomes. Developmental and hormonal profiles for each SCT condition will be modeled, and the association between early risk factors and outcomes at 3?4 years of age will be tested. Further, the sensitivities of common primary care DD and ASD screeners will be calculated for each condition using direct developmental test results as gold?standard. Impact: Prospective study of the natural history of prenatally diagnosed infants with SCT will allow investigation of important questions to inform newborn screening considerations, such as the interplay between early hormonal profiles and developmental outcomes. Results will be immediately relevant for counseling and establishing evidence?based care guidelines for the rapidly increasing rate of SCT diagnoses from cfDNA screening. Results will serve as the basis for ongoing longitudinal studies of health and psychological outcomes of SCTs through the lifespan.